21 replies to this topic

[jprocess]

Dear All,
I have 2 questions regarding to multi-stage compression system.I should be grateful if experienced engineers would answer to my questions:
As you all know,we have intercooler after each compressor.What should be the best temperature for the intercooler outlet process fluid?
I read in a technical document that it should be 5 degrees higher than fluid dew point.Is there any concept behind of setting the intercooler outlet temperature to this value?
Also,What is the best arrangement for anti surge protection piping for multi-stage compression?That is,where is the best location to have branch for recycle piping?
One experienced engineer said to me that it should be as short as possible so it is better to have separate baranch for each compressor beginning from the intercooler discharge to the inlet of compressor k.o. drum.
Thanks in advance.
Cheers.

[Art Montemayor]

Jprocess:

Before any engineer starts to talk or write about “compressors”, he/she should be specific about WHAT TYPE of compressors. There are big differences between the various type as to design, calculations, efficiencies and, most important, mechanical components and workings.

Again, I am forced to make a guess at what you mean so I will have to assume you are referring to CENTRIFUGAL compressors. You really are asking 3 questions, not 2:

1. The best gas temperature to have at the intercooler outlet (subsequent stage inlet) is the coldest temperature you can get it down to within the mechanical design parameters of the machine: what temperature the metals can safely take and NO liquid particles included in the gas. This, of course also assumes you are dealing with “wet gas” – i.e., gas that is saturated at its temperature and pressure. You also failed to state that you are compressing saturated gas, but I also have to assume that you are.

2. The only concept in instructing you to set the suction temperatures “5 degrees higher than fluid dew point” is the assumption that you will be too dumb to realize that you might be having bad vapor-liquid separation before entering the subsequent stage and also that you might be having condensation inside the pipe that is leading to the subsequent suction port. I’ve never seen or had this experience. My comment is that this is the latent fear of many engineers who haven’t had in-depth, hands-on experience with compressors – such as mechanically taking them apart, piece-by-piece, and putting them back together as originally made or mechanically modified. I’ve done both and haven’t witnessed any suction line condensation. Of course, if you feel you want to be super-safe, you are free to do so – but this requires line heaters and controls. Your discharge temperatures will also be higher.

3. Recycle gas doesn’t necessarily produce gas “surge” and also doesn’t necessarily involve any so-called “surge protection”. If you are referring to centrifugal machines, then the machine usually comes with a conventional recycle surge control valve that protects the machine from having to confront non-designed capacity situations. This is a big handicap of the centrifugal machine. The reciprocating type doesn’t have to contend with this factor. The best arrangement for anti surge protection piping for multi-stage compression is exactly where and how the manufacturer (and the packager) recommend you to do so – usually the way and the manner they supply the machine.

An “experienced” engineer might give you his opinion on how to route and install the recycle piping, but will he personally guarantee the results? And if he/she does, what is his/her guarantee worth? The plain and simple truth is that you are far more protected when you apply the guidelines and rules set forth by the people who built and supplied you the machine – the manufacturer. This is just common sense engineering.

[jprocess]

Dear Mr.Montemayor,
Thanks a lot for your valuable comments.
I should add some points:
1.The compressors are centrifugal type and electrical driven.Because we must include loss of individual feeds in our design and also to obtain flexibility with a high percent the compressors will be variable speed.
2.The gas feed is completely saturated.
3.In fact this is a Gas Gathering and NGL Recovery project.So,because the downstream unit of compression station is a NGL one, the liquid removal in gas compression unit should be as small as possible.
So we need a logical margin above gas dew point.In contrast,the more the compressor feed temperature,the lower the compressor efficiency.We can not use 5 degrees margin because the feed gas is naturally heavy and consequently its dew point is high.So what is your recommendation for margin?
4.But about anti surge:
Our company have had another similar project.In that project the vendor recommended to use individual anti surge for each compressor(from each inter-cooler discharge to related compressor suction drum) because it provides a shorter recycle path and a faster response against surge.But in this new project the Consultant that is responsible for basic design stage has used a different configuration for anti surge line.It begins from the last k.o drum that is located after last compressor.Any hint?
Thanks in advance.
Cheers.

[asade abiodun]

Art,

During sizing of multistage centrifugal compressor, i told me that i can set the outlet temperature of the interstage cooler to the inlet feed gas stream temperature from the feed inlet scrubber.

Based on your experience, is the decision okay for my design.

Regarding surge control, are you saying that the manufacturer is in the best position to determine the position of the surge line. Further clarification required!

Thanks in anticipation for your responce.

Asade Abiodun

[palusa]

Jprocess

based upon my experience, the selection of the design insterstage-cooler temperature results from a balance between the cost of cooling (I assume here air cooling is used) and the compressors power requirement. The colder the outlet of the interstage cooler, the lower the compression power but the higher the cost of aircooling.

I don't know where the rule of "5deg above dewpoint" comes from, however I've seen several times compression plants where, under particular circumstancies (always in very low ambient Temp locations and during gas feedstocks composition's swings at mol-weights much higher than the design) partial condensation to take place downstream interstage aircooler.
In this case the liquid MUST be separated in the subsequent stage inlet separator...so ensure that the separator is consistently designed for and suitable to remove liquids otherwise you'll run in serious trouble...

Finally, I strongly support your previous project's antisurge philosophy (at least for centrifugal compressors) ... antisurge lines as short as feasible and for each individal stages (and taken upstream the interstage cooler and routed to previous stage separator inlet).
When, as i guess is your, the compression train has multiple stages with multiple compressor casings, each compressor behaves according to its own curves and its surge limits and should be protected individually.

Proceed with caution with "single" antisurge ... require your consultant to elaborate the dynamic simulation results on this specific respect.


Good luck.

_Lf_

[djack77494]

For a high speed centrifugal compressor, you may want a hot AND a cold recyle. The hot recycle valve should be as close to the compressor discharge as possible. It is designed to very quickly "dump" excessive pressure from the discharge side of the compressor. The cold recycle circuit could handle longer term or even continuous partial recycling of your gas stream. It would originate downstream of the stage cooler, and could be slower acting. Both recycles would return gas to the upstream Scrubbers/Knock Out Drums.

For an existing system, you would want to cool to as low a temperature as practical. For a new system, economics would dictate the cool gas temperature. You shouldn't need to worry about the dewpoint since any (presumably small) quantity of condensed phases would be removed by the Scrubber.

I support the philosophy of separately protecting each stage UNLESS there are no interstage additions or withdrawals AND an analysis of the system (preferably by the vendor) indicates that a single recycle is acceptable.

Good luck,
Doug

[jprocess]

Thanks a lot for your valuable comments.
But 2 additional questions:
1.What does "dynamic simulation" of compression loop mean?
2.The anti surge control valve seems to be an special one with booster.Am i right?Who have experience about it?
Thanks in advance.
Cheers.

[palusa]

Jprocess,

1) Dynamic symulation is normally carried out for large centrifugal compression train in critical service. Part of it is the evaluation of the response of the overall compression system (including sensors, transmitters, controllers, actuators and valves and taking into consideration actual piping, cooler and separators configuration) to disturbance. One scenario is of course the antisurge valve opening: as a function of the conceptual and physical arrangement of the antisurge there are different responses in terms of time of opening and pressure build-up.

2) Normally antisurge system (both valves and their control system) is part of the scope of supply of the centrifugal compressor manufacturer.

Hope this could help.

_Lf_

[djack77494]

As you surmised, anti-surge control is not routine or "off the shelf". In fact there are suppliers who specialize in the design of such systems, such as CCC (I think Compressor Controls Corp.). The anti surge system needs to respond VERY quickly to any incipient surge conditions to protect the compressor. You cannot use ordinary components such as sensors, control valves, etc. since they respond too slowly.
Doug

[Padmakar Katre]

Hello sir, I have a article regarding control of a centrifugal compressor specially antisurge control i hope it can help you.

[Padmakar Katre]

I have an experience of how the antisurge protection works to a compressor
now i will tell abt our plant we a single stage centrifugal compressor which is turbine driven it has one motorised operated valve in outlet which goes to reactor via one exchanger and outlet of the reactor there is one motorised operated valve and the outlet goes to scrubber now there is one another line from compressor discharge which goes directly to the scrubber bypassing the reactor and the exchanger as soon as there is any problem in down stream to reactor side the antisurge gets open as well whenever turbine trips the antisurge valve opens as it is the requirement in start-up of the turbine. i hopw you can get an idea of how the antisurge works

[Art Montemayor]

Padmakar:

That is a nice paper on Centrifigual Compressor control you have posted. However, for it to be of any value, it must be identified in order to qualify it or to be able to refer to it in the future.

Can you please cite the author, the date, the publication, the references, or the journal you obtained it from?

Thank you.

[gvdlans]

Art,

I did a Google search and found that the text is copied from a paper published on the Micon website: http://www.miconsyst...ers/centrif.htm

It was written by:

Roman Rammler P.E.
MICON
Houston,Texas

Dale Lupfer P.E.
Consultant to MICON
Houston,Texas

I could not find a date of publication.

[Art Montemayor]

Guido:

Thanks a bunch! You're a Pal, indeed!

Art Montemayor

[gvdlans]

Guido:

Thanks a bunch! You're a Pal, indeed!

Art Montemayor

My pleasure Art. I am glad I could do you a small favour in return...

Guido

[Steve McGahey]

For a high speed centrifugal compressor, you may want a hot AND a cold recyle. The hot recycle valve should be as close to the compressor discharge as possible. It is designed to very quickly "dump" excessive pressure from the discharge side of the compressor. The cold recycle circuit could handle longer term or even continuous partial recycling of your gas stream. It would originate downstream of the stage cooler, and could be slower acting. Both recycles would return gas to the upstream Scrubbers/Knock Out Drums.

Why should the Hot Gas Bypass return gas to the upstream KO drum? I can tell you that doing this reduces the effectiveness of the hot gas bypass, because you want it to do TWO things, not one. You mention the depressurisation of downstream of the compressor, but you don't talk about pressurising the upstream section, which also helps save the compressor from surge.

I think I've only seen one design where the hot gas bypass discharges into the KO drum - all the other ones I've been involved in provide a direct connection between discharge and feed. Can someone enlighten me why you would make a hot gas bypass go via the KO drum?

Thanks,
- Steve

[djack77494]

Steve,
You seem to have a problem with the anti-surge strategy I described, but I'm not sure what your objection is. For the very rapid response, especially regarding the all-important dropping of discharge pressure to get away from the realm of dangerous surge, you want to dump pressure from as close to the compressor's discharge flange as possible. The gas there is hot, having just undergone compression. A large continuous recycle of this hot gas stream would be objectionable as the hot gas recycles back to the inlet scrubber and then reenters the compressor. Hence the COLD recycle line.

Now, to directly address your question, the BEST place, and quite often the only place to send this gas is to the suction of the compressor. That is what I'm doing. Where is the Scrubber, but immediately upstream of the compressor - at its suction. Sending the gas here is also helping to pressurize the suction. Are you suggesting that you bypass the Scrubber and enter the suction piping downstream of the Scrubber? If so, I would object on the following grounds: Generally the bypass line is stagnant. If exposed to hot saturated or nearly saturated gas, there is the possibility for some condensation to occur. I would not want to chance having any liquid enter my compressor, so would be very inclined to route this stream back thru the Scrubber.
Doug

[Steve McGahey]

Hey Doug,

Now, to directly address your question, the BEST place, and quite often the only place to send this gas is to the suction of the compressor.

Are you suggesting that you bypass the Scrubber and enter the suction piping downstream of the Scrubber?

Yes, that's exactly what I'm suggesting. My problem, is that you talk about returning flow via the knock out (KO) drum INSTEAD of pushing it straight into the compressor suction.

If so, I would object on the following grounds: Generally the bypass line is stagnant. If exposed to hot saturated or nearly saturated gas, there is the possibility for some condensation to occur. I would not want to chance having any liquid enter my compressor, so would be very inclined to route this stream back thru the Scrubber.
Doug

That's a fair argument.

But, then you're moving the returned flow "far" (in terms of process volume, not point-to-point distances) from the compressor suction by including the KO drum volume.

A HGBV is trying to save the compressor from surge by depressuring the discharge, and re-pressurising the suction. I think that the re-pressurisation of the compressor suction is going to be faster if you thow the recycled gas right into the compressor suction flange than if you stick it into the KO drum. So, I'm talking about a couple of short pieces of pipe, with a valve between them.

Now, you say there's a problem with condensation - which I can appreciate. How about these points:

(1) Is there some way to allow the hot gas bypass to drain naturally? I suppose that will depend on how the vendor constructs their compressor. What I'm suggesting is a straight-line, so this would be dictated by where the vendor sticks their suction and discharge flanges. Perhaps a better design which would alleviate your concern would have the high pressure end of the hot gas recycle travelling vertically to the bypass valve. This would allow for gravity-drainage of the hot gas bypass line if condensation forms.

(2) Since the effectiveness of a hot gas recycle can come down to performance over a time period of milliseconds, surely every additional delay is going to be costly? Isn't it best to get rid of all the intervening volume between compressor discharge flange and compressor suction flange?

(3) Which would be worse (not a rhetorical question):
(3a) a small bit of condensation for a small period of time entering a compressor suction, while the compressor is either shutting down, or being saved from surge, OR
(3b) a delay in the overall de/re-pressurisation at the compressor discharge/suction?

I would imagine that if point 3a is not going to cause real damage to the compressor, then the cost savings in getting a smaller hot gas bypass valve (since these things need to move very quickly they're quite expensive) might be significant.

My angle on all of this is when I'm doing compressor surge studies I sometimes see these hot gas recycle via KO drum designs, and since they take longer to pressurise the compressor suction, they tend to need larger, more expensive bypass valves. Conversely, the direct discharge-to-suction designs don't need such large valves.

As always, the issue comes down to balancing risk versus investment to alleviate risk. I'm sitting in the "damage due to condensation is not so risky because it (condensation) can be minimised" camp.

Since it's been something I've wondered about for a long time, I would be really interested to hear your (and anyone else reading this) thoughts on the points I raise above.

Cheers,
- Steve.

[djack77494]

Steve,
You present some strong arguments and I do not disagree with the validity of the points you raise. I guess my bias, from past experience, is to prefer routing the HGB to the Scrubber rather than directly into the compressor suction. I'm told that between depressurizing the compressor discharge and pressurizing the compressor suction, the former is more important in preventing surge. Thus I have accepted the strategy as it was presented to me. On a fairly recent project, I spoke with a compressor expert about these matters and he preferred going into the Scrubber for the reason I mentioned; he didn't seem too concerned with very rapid suction pressurization.

I don't think we disagree so much as just have different backgrounds and experiences. Both points of view are valid and the decision between them is a compromise.
Doug

[Steve McGahey]

I don't think we disagree so much as just have different backgrounds and experiences. Both points of view are valid and the decision between them is a compromise.
Doug

I agree. Thanks for the explanations - it's not always possible to talk directly with the person responsible for design variations (when you're an external consultant).

This thread has been good - I feel like we've managed to close out something I've been wondering about for a while.

Cheers,
- Steve.

[Satyajit]

Hi everybody!
It was a very interesting topic. In many process industries ,antisurge issue practically plays a big role in plant safety and reliability.
Apart from all the important discussions done so far, I would like to add few things to improve the anti surge control.
In a continuous plant operation, trip of downstream unit trips the compressor due to overspeed as a result of high back pressure.
To prevent compressor trip or upstream plant upset , one may incorporate following whether in a running plant ( of course in an oppourtunity) or in a new project.
a. Give a pre-set value to open kickback .
b.To reduce the compressor speed with a pre-set value.
c. Open the discharge vent also with a preset value based on flowrate and plant load.
d. To check/modify recycle valve in case of revamping a plant for higher capacity.

Cheers!
SM

[bernath]

Hello sir, I have a article regarding control of a centrifugal compressor specially antisurge control i hope it can help you.

Hi Padmakar,

what is the title of this article? Have you ever posted it? I can't find it here.

thank you
regards,
bernath

Edited by bernath, 14 April 2011 - 09:01 PM.